1. Field of the Invention
The present invention relates to a scanning image display apparatus for observing a two-dimensional image formed by two-dimensional scanning with a modulated light beam based on image information, and is suitable for use in, for example, an electronic view finder which is mounted on an image pickup apparatus such as a digital still camera or a video camera, or a head mount display.
2. Related Background Art
Up to now, there have been proposed image display apparatuses without a two-dimensional display device which is so called a flat panel, such as a transmission liquid crystal element, a reflection liquid crystal element, and an organic EL element. In the image display apparatuses, a predetermined surface is two-dimensionally scanned with a light beam by a scanning member and an image is displayed for observation.
For example, there have been proposed scanning image display apparatuses of a retina direct image drawing type, in which a light beam emerged from a light source is scanned two-dimensionally to directly form an image on the retina of an observer (U.S. Pat. No. 5,467,104 and U.S. Pat. No. 5,701,132). In the image display apparatuses of a retina scanning type which is disclosed in U.S. Pat. No. 5,467,104, the light beam which is two-dimensionally scanned is imaged onto a intermediate image plane and then imaged onto the retina of the observer through an ocular optical system (eyepiece), thereby forming a two-dimensional image on the retina thereof. For the image display apparatus, it is necessary to scan light at very high speed, so a very small scanning member is used for a scanning portion such as a mirror for scanning light. An example of the scanning member of the scanning image display apparatus which is to be used is a mirror which is compact, lightweight, and operable at high speed. The mirror is produced using a microelectro mechanical system (MEMS) technique.
The light beam scanned in the image display apparatuses is very thin, so that the diameter of the light beam in a pupil position of the observer becomes very small.
A method of enlarging the small diameter of exit pupil is disclosed in U.S. Pat. No. 5,701,132. According to the method disclosed in U.S. Pat. No. 5,701,132, a light diffusing means such as a lens array or a diffusion plate is disposed on an intermediate imaging surface having a formed curvature, and a scanned beam is made incident on the light diffusing means. Therefore, when the beam passes through the light diffusing means, the spread angle of the beam increases.
Japanese Patent Application Laid-open No. 2001-004955 or Japanese Patent Application Laid-open No. 2001-194617 discloses an image display apparatus using a prism member which has a rotationally asymmetric surface and is made of a material whose refractive index n is equal to or larger than 1. When the prism member is used, an optical path is folded to reduce a size of an optical system and a length of the optical path is set to be n times (n>1) an actual light traveling distance to ensure a long optical path length. However, in the optical system, the intermediate imaging surface is included in the prism member, so the light diffusing means disclosed in U.S. Pat. No. 5,701,132 cannot be used.
Japanese Patent Application Laid-open No. 2004-045496 discloses an optical system using a prism filled with a medium whose refractive index is larger than 1, serving as an optical system for imaging a scanned light beam on a surface to be scanned. In the optical system, an image formed on the surface to be scanned is observed through an ocular optical system including a diffusion plane disposed to the surface to be scanned.
For example, a structure including a lens (refractive optical element) or a structure including a prism member and a lens can be applied to the optical system for imaging the light beam scanned by the scanning member onto the intermediate image plane. In the case of the optical system using the refractive optical element, when remaining chromatic aberration is large, image quality deteriorates. In order to correct the chromatic aberration of the optical system, optical elements having different dispersions are necessary. Therefore, the entire optical system becomes larger in size.
When the light beam from the light source is incident on the scanning member at a large angle, a so-called scanning distortion is caused on an image formed on the surface to be scanned.
By employing a structure in which the surface to be scanned is scanned simultaneously with a plurality of light beams emitted from a light source having a plurality of light emitting points, a two-dimensional image can be formed at high speed.
Here, assume that the plurality of light emitting points are linearly or two-dimensionally arranged and each of light beams from the light emitting points is converted into a light beam having a predetermined shape (beam diameter is adjusted) to be made incident on the scanning member. In such a case, when a suitable optical system is not constructed in order to guide the light beams from all the light emitting points onto the surface to be scanned while desirable optical performances thereof are maintained, the quality of a formed image significantly deteriorates.